The term "disinfection", as applied to municipal wastewater, refers to a reduction in the population of pathogenic organisms (generally bacteria species). Since several different bacteria species are known to be present in wastewater, an indicator organism (such as the bacterium Escherichia coli which is known to exist in human fecal matter) for which specific monitoring methods are available is used to monitor disinfection. The effluent from a wastewater treatment plant (WWTP) is considered to be "disinfected" if the number of indicator organisms per volume of effluent falls at or below a preset guideline. These guidelines are typically included as one of a set of effluent limitations in a National Pollution Discharge Elimination System (NPDES) Permit specific to a single WWTP. Permits are issued at the State level by a regulatory body within the State, with the Environmental Protection Agency having oversight authority. A typical NPDES Permit may mandate daily testing for fecal coliform bacteria and may stipulate limits such as "less than 200 per 100 mL effluent" for the geometric mean of 30 days of tests and such as "less than 400 per 100 mL effluent" for the geometric mean of 7 days of such tests. Since the early part of the twentieth century chlorine has been commonly used to disinfect wastewater. Although several chlorine-releasing chemicals may be used to accomplish this, a more typical method is for chlorine gas to be added to the wastewater directly. This is usually one of the last or the last treatment in a municipal WWTP.
Chlorine must be added in an amount sufficient to satisfy the "chlorine demand" of the water being treated. When chlorine is added to wastewater at low concentrations, a fast reaction occurs with various organic species, including naturally occurring chemicals such as fulvic and humic acids. The initial amount of chlorine which reacts in this fashion is said to be satisfying the chlorine demand. The demand of different waters will vary depending on the amount(s) of organic species present. Once the demand is satisfied, addition of more chlorine will result in a chlorine residual which is detectable by various analytical means. It is the residual chlorine which is available as an oxidizing biocide to disinfect the water. The level of chlorine residual required to disinfect WWTP effluent varies from one plant to another and within the same plant due to variations in the specific wastewater being treated Some plants may be able to disinfect with a chlorine residual of 0.1 parts per million (ppm) whereas others may require greater than 2.0 ppm. Typical levels of chlorine residual required for disinfection would be in the range from 0.3 to 1.0 ppm.
Chlorine residual limits are generally included in NPDES Permits. Until recently, chlorine residual limits have been relatively high and plants have generally not had problems maintaining disinfection. In 1984, the EPA established a National Municipal Policy which required WWTP's to comply with provisions of the Clean Water Act. Under the Clean Water Act, the States have the responsibility to regulate chemicals which are discharged from WWTP's. One chemical which is under scrutiny at the present time is chlorine. Specific concerns about chlorine stem from fish toxicity, fish avoidance of the zone in which chlorine is discharged, formation of chloramines (reaction products of chlorine and ammonia which are also toxic to fish and which persist longer than chlorine), and formation of halogenated methanes as a result of chlorine addition. As a result of these concerns, several States have proposed that the level of residual chlorine in WWTP effluents be considerably lower. In Michigan, for example, the level which was traditionally acceptable was 1.0 ppm. It is now proposed that the acceptable limit be reduced to 0.036 ppm. Most, if not all, WWTP's would not be able to meet the requirements for disinfection (which are not expected to change) with chlorine residuals below 0.036 ppm.
In an attempt to reduce the level of chlorine in the effluent, some WWTP's are adding sulfur dioxide feed systems. Sulfur dioxide, when added to the effluent, will react with the residual chlorine rapidly and cause the measurable residual to diminish to non-detectable. There are numerous disadvantages to this technique, including the capital cost of installing the feed system, the on-going chemical cost, the operating and maintenance expense, the possible effects on the chemistry of the effluent, the reporting requirements for leaks, and the relative lack of data on the environmental impact of sulfur dioxide.
Ultraviolet disinfection and ozonation as a means of disinfection have both been proposed. While both are theoretically possible, neither has proven to be reliable in WWTP's. Moreover, the cost of either technique is very high, involving both high capital expenses and high operating and maintenance costs.
Polishing ponds have also been proposed as a means to meet new requirements. These are merely holding basins in which wastewater effluent is exposed to the environment for some time period after disinfection and the residual chlorine is allowed to dissipate naturally. Use of this technology requires considerable space, which could necessitate the purchase of land. In addition, installation of the polishing pond for even a small WWTP is costly. This method will also result in the re-growth of bacteria prior to discharge of the effluent.
An economical method which would allow WWTP's to lower the addition of chlorine to a point where chlorine residual would be acceptable from an environmental point of view while simultaneously maintaining the levels of disinfection required in their operating permits is highly desirable.